I think this is less of something that "Nintendo isn't pushing enough" and more of a consequence of how 80s/90s videogames were made. I think
Ben Holt
summed it up nicely.
I don't know much about DQ (XI S is looking to be my first one), but Eleven and Eight don't have female alts, apparently. So it would be a bit weird in my eyes for only 2 of the 4 Heroes to have female versions (which also means less nods to other DQ games). Plus that's two more models with full voice acting, more work time and possibly, more money to Square.
People really underestimate the amount of time and effort these things could take - having 4 alts at all was already a huge feat for me. I mean, I wanted a Ms. Pac-Man alt as much as the next guy, but if Sakurai chose to focus on other references and aspects for other characters... so be it.
Also, here's food for thought... let's compare the two biggest Capcom names in speculation now: Jill Valentine and Phoenix Wright.
The first is obviously female, but her series has a bigger male demographic (around 77%, though the only clue I could find had a broken link, so take it with a grain of salt). Meanwhile, the other is a male character, but his series is known for having a high female player ratio (though I couldn't find any definitive sales data from Capcom).
So hypothetically, which is the "right" choice? The female character? Or the character that could potentially attract a larger female base? Personally, I don't think there
is a right answer. I think the most important point here is that new characters bring new things - and new people - to Smash - the
how may or may not be a factor.